Enhanced ductility in coarse-grained Al-Mg alloys
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L
Table L Alloy Compositions in Weight Percent
INTRODUCTION
ENHANCED tensile ductilities, of up to 300 pct, have been observed previously at warm temperatures in several coarse-grained, solid solution alloys of aluminum containing magnesiumY~] Such high tensile elongations are especially remarkable, because none of these materials exhibit the significant grain-boundary sliding behavior that is typical of most fine-grained, traditional superplastic metallic alloys. The capability of coarse-grained materials to exhibit high tensile ductilities is of great interest from the viewpoint of creating materials which do not require expensive processing to achieve high formabilities and are therefore economical to fabricate. High formability is obviously important in forming operations, because it permits a costeffective approach to the manufacturing of complex components through a reduction of required stamping, machining, and joining operations. The enhanced tensile ductilities observed in these coarsegrained AI-Mg alloys are evidently not the result of a classic superplastic deformation mechanism but rather the result of a solute-drag controlled creep mechanism,tg,~~ Alloys which exhibit such solute-drag or viscous-glide controlled creep behavior, referred to here as Class I alloys after the original definition by Sherby and Burke, tt~] exhibit an inherent high strain-rate sensitivity value of m ~- 0.33. By contrast, pure aluminum and aluminum alloys with a low magnesium content do not exhibit high tensile ductilities at warm temperatures; instead, they exhibit either pure metal or Class II alloy behavior with a low strain-rate sensitivity of m ~ 0.2.[H] Increasing values of m generally produce increases in tensile ductility for many materials,t12] By contrasting the tensile and torsional behavior of several aluminum alloys with differing magnesium concentrations, McQueen and Kassner showed that the enhanced tensile ductility observed in the Class I AI-Mg alloys was primarily a result of the high strain-rate sensitivity values of these alloys,tg] This is
ERIC M. TALEFF, Assistant Professor, is with the Department of Aerospace Engineering and Engineering Mechanics, The University of Texas at Austin, Austin, TX 78712-1085. DONALD R. LESUER, Group Leader/Materials Engineer, Manufacturing and Materials Engineering Division, and JEFFREY WADSWORTH, Associate Director, Chemistry and Materials Science Directorate, are with the Lawrence Livermore National Laboratory, Livermore, CA 94550. Manuscript submitted October 25, 1994. METALLURGICAL AND MATERIALSTRANSACTIONS A
Alloy
Mg
Mn
Si
Fe
Cu
Zn
Ti
I II III IV V
1.02 2.52 4.05 5.51 6.64
0.50 0.46 0.46 0.47 0.48
0.028 0.034 0.039 0.043 0.043
0.022 0.022 0.023 0.023 0.023
0.001 0.002 0.002 0.002 0.002
0.015 0.017 0.024 0.021 0.022
0.001 0.001 0.001 0.001 0.001
in contrast with the case for torsional ductility, in which recovery processes, especially dynamic and geometric-dynamic recrystallization, were controlling and pure aluminum exhibited higher ductility than its
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